Make the DNSBL used for proxy blocking configurable. This is better than hard…

Description

Make the DNSBL used for proxy blocking configurable. This is better than hard-coding, but a lot of the internal variables still call it SORBS, and unfortunately this will need to continue - as I don't want to break existing configurations. Updated the messages for SORBS blocking to refer generically to a DNSBL, rather than to any particular one.